How do I convert a font% size to style-delta% size?

[benknoble]

A style-delta% can only 'change-size to values of a byte?, but it is documented as taking a "size." Meanwhile a font% reports sizes as (real-in 0.0 1024.0). (A Racket byte? is 0 to 255.)

I'm not even sure how to interpret the values, so it's not clear how to convert one to the other. (I'm already using 'change-size-in-pixels to make sure the style-delta% and font% interpret the size the same way, but they don't speak the same contract…)

I tried using a size of 1, an additive factor of 0, and a multiplicative factor of the font's size, but that produced the wrong results (too big) on normal-control-font, which has a size of 13.0.

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Context: I'm trying to convert a font% to a style-delta% so that the different fonts in my application are consistent. All the other properties seem straightforward to translate. Here's the current version of the function, and the results using normal-control-font as the base font (minus the size changes, of course):

(define (font->delta f)
  (define d (new style-delta%))
  (define size-in-pixels? (send f get-size-in-pixels))
  (define size (send f get-size))
  (define family (send f get-family))
  (define face (send f get-face))
  (define style (send f get-style))
  (define weight (send f get-weight))
  (define underline (send f get-underlined))
  (define smoothing (send f get-smoothing))
  (send* d
         (set-delta 'change-size-in-pixels size-in-pixels?)
         #;(set-delta 'change-size size)
         (set-delta 'change-family family)
         (set-face face)
         (set-delta 'change-style style)
         (set-delta 'change-weight weight)
         (set-delta 'change-underline underline)
         (set-delta 'change-smoothing smoothing))
  d)

(I'm also hoping to figure out how to scale the pict? in the text so it's the same size, but that's unrelated to this, I think.)

[benknoble]

It seems the docs are wrong; 'change-size takes any exact-integer?; using exact-truncate is enough for me for now.